All molecules vibrate and rotate at characteristic frequencies in the electromagnetic spectrum. These vibration/rotational frequencies cause asymmetric molecules such as CO2 and H2O, but not symmetric molecules like N2 or O2, to absorb light at very specific wavelengths, particularly in the infrared. The NDIR gas measurement technique targets these characteristic absorption bands of asymmetric molecules of gases in the infrared for their detection. The term “non-dispersive” which actually implies “non-spatially-dispersive” as used herein refers to the apparatus used, typically a narrow-band infrared transmission filter instead of a spatially-dispersive element such as a prism or diffraction grating, for isolating for the purpose of measurement the radiation in a particular wavelength band that coincides with a strong absorption band of a gas to be measured.
The NDIR technique has long been considered as one of the best methods for gas measurement. In addition to being highly specific, NDIR gas sensors are also very sensitive, relatively stable and easy to operate and maintain. In contrast to NDIR gas sensors, the majority of other types of gas sensors today are in principle interactive. Interactive gas sensors are less reliable, short-lived and generally non-specific, and in some cases can be poisoned or saturated into a nonfunctional or irrecoverable state.
Despite the fact that interactive gas sensors are mostly unreliable and that the NDIR gas measurement technique is one of the best there is, NDIR gas sensors still have not enjoyed widespread high volume usage to date. The main reasons for this can generally be attributed to their high unit production cost, relatively large size and output drifts over time.
Just about all gas sensors ever designed and manufactured to date, irrespective of what technology is being employed, invariably have significant output drifts over time. While NDIR gas sensors can be recalibrated as part of a periodic maintenance program or service, the cost of such recalibration has prevented NDIR gas sensors from being widely adopted for many applications.
Recently the present author in U.S. Pat. No. 8,143,581, the disclosure of which is specifically incorporated by reference herein, advanced the teaching of an Absorption Biased (“AB”) NDIR Gas Sensing Methodology which is capable of significantly reducing sensor output drifts over time. This AB methodology can be reviewed briefly as follows. First of all, this methodology is based upon a conventional Double Beam Configuration Design for NDIR gas sensors. Two channels or beams are set up, one labeled Signal and the other Reference. Both channels share a common infrared source but have different detectors, each of which is equipped with the same or identical narrow band-pass filter used to spectrally define and detect the target gas of interest. Both detectors for the two channels share the same thermal platform with each other and also with the sample chamber and the common infrared source mount for the sensor. An absorption bias is deliberately established between the Signal and Reference channels by having the sample chamber path length longer for the Signal channel than that for the Reference channel. By so doing, the detector output of the Reference channel is always greater than that of the Signal channel when there is target gas present in the sample chamber. This is due to the fact that there is more absorption taken place in the Signal channel because of its longer sample chamber path length. By applying this absorption bias between the Signal and Reference channels, one is able to calibrate the sensor even when both channel detectors have the same and identical narrow band-pass filters.
The Absorption Biased (“AB”) NDIR gas sensing methodology addresses directly a serious technical weakness never recognized by engineers for decades in the design of the conventional and the most popular Double Beam Configuration NDIR gas sensors. In this conventional design, a Signal channel and a Reference channel are set up between the shared blackbody source and two infrared detectors. The Signal channel detector is equipped with a narrow band-pass filter which is used to spectrally define and detect the target gas of interest. The Reference channel detector is on the other hand equipped with a narrow band-pass filter which is located spectrally away from the absorption band of the target gas of interest and is also neutral to the absorption bands of all other common gases present in the atmosphere. According to the rationale for the design of the Double Beam methodology, the addition of a Reference channel operating at a different wavelength off the Signal channel absorption one and then processing the ratio of the signals of the two channels as the sensor output will eliminate or reduce many error-causing factors common to both channels. While the Double Beam design is no doubt superior to the Single Beam one, it has overlooked one major technical problem for this design which for decades has caused output drifts over time for NDIR gas sensors.
The problem referred to above has to do with the aging of the blackbody source common to both the Signal and the Reference channels. As the blackbody source ages, its operating temperature and therefore output will inevitably change, not only in magnitude (radiation intensity) but also in spectral content, as dictated by Planck's radiation law for blackbodies with different temperatures. Since the Signal channel filter and the Reference channel filter each passes radiation of a different wavelength from the blackbody source to their respective detectors, their signal ratio will change as the spectral content of the source changes causing the sensor output to inevitably drift over time. The Absorption Biased (“AB”) methodology discussed earlier above addresses this problem head-on by applying the same spectral narrow band-pass filter for target gas detection to both the Signal and the Reference channels. An absorption bias is then applied to the Signal channel by making the sample chamber path length associated with it longer than that associated with the Reference channel. In this way even though both the Signal and the Reference channels have the same spectral filter, the sensor can still be calibrated for the amount of target gas present in the sample chamber because of the absorption bias applied.
The present invention advances a more reliable NDIR gas technique and sensor which will remain more stable over time, while also reducing unit production cost and size. The present invention does this by improving upon the AB methodology.